For the optimal operation of an internal combustion engine by means of a control apparatus, it is necessary to detect the particular injection start of the injection valves of the engine. For this purpose, a trigger circuit is used which evaluates the electric signal of a transducer coacting with the particular injection valve.
Triggering takes place by evaluating the amplitude of the signal waveform of the periodic signal emitted by the transducer. The triggering takes place by means of a comparison with a reference value and is utilized as a time point of the injection start. The time point is transmitted to a computer of the control apparatus which compares the detected injection start to a predetermined characteristic field value of the control apparatus with a control taking place via a timing device in such a manner that the injection time point always takes on the optimal value stored in the characteristic field. In this way, the quality of the exhaust gas can be improved and an economical consumption of fuel is obtained.
One problem is that the magnitude and the shape of the signal greatly fluctuate in dependence upon the many engine parameters. Accordingly, the amplitude of the signal changes with the speed of the engine. This applies also to the signal waveform and especially to the signal flanks. It has been determined that the injection start can be determined from the signal with adequate certainty when triggering is always utilized when the signal amplitude of a monitored period of the signal amounts to approximately 20% of the maximum signal amplitude of the previous signal period. This presupposes that a reference value of the previous signal period is available which amounts to approximately 20% of the corresponding maximum signal amplitude. If the signal reaches this reference value in the monitored period, then the trigger signal is emitted and transmitted to the computer of the control apparatus which carries out the comparison with the characteristic value and the control already mentioned. It is a disadvantage here that the signal of the transducer includes bounce pulses under certain circumstances having amplitude peaks which are greater than the amplitude of the actual useful signal and have nothing to do with the wanted injection start. In this way, the result can be falsified.
Published German patent application DE-OS 26 14 947 discloses a switching arrangement for detecting the position of a rotating part. An inductive transducer is provided which emits an alternating voltage for each corresponding rotation of the part. This alternating voltage is rectified and applied to an integrating capacitor. The integrating capacitor is connected to the one input of a comparator. The alternating voltage is further applied to a zener diode via a series resistor and the zener diode is connected to a further input of the comparator. The above-mentioned integration leads to the condition that incoming disturbance pulses contribute to an increase of the threshold voltage of the comparator in correspondence to their voltage-time area and in this way are considered.